1. Field of the Invention
The present invention relates to sheet stacking apparatuses and image forming apparatuses, and in particular to sheet stacking apparatuses in which sheets are selectively stacked onto a plurality of sheet stacking units.
2. Description of the Related Art
With the advancement in technology, recently developed image forming apparatuses form images on sheets at an increased image forming speed. With such an increase in the image forming speed, the speed of discharging sheets from the body of such an image forming apparatus is also increasing. For the purpose of aligning and stacking a large number of sheets that are discharged at a high speed, there are some image forming apparatuses that each include a large-capacity stacker apparatus, as a sheet stacking apparatus, disclosed in Japanese Patent Laid-Open No. 9-255213.
Referring to FIG. 24, a known sheet stacking apparatus 1 includes two stacking units, a first stacking unit 3A and a second stacking unit 3B, that can be individually raised and lowered while carrying sheets thereon. In the sheet stacking apparatus 1, if a redirecting tab 16 is turned upward (to a position shown in broken lines), each sheet that has been conveyed thereto through a first pair of sheet discharging rollers 15 can be stacked onto the first stacking unit 3A on the left side. As the number of sheets stacked on the first stacking unit 3A increases, the first stacking unit 3A is lowered. When a full-state detection sensor 19 detects that the first stacking unit 3A is full of sheets, the lowering of the first stacking unit 3A is stopped. Then, the redirecting tab 16 is turned downward (to a position shown in solid lines). Each sheet that has been conveyed thereto is guided to a second pair of sheet discharging rollers 18 on the right side and, through the second pair of sheet discharging rollers 18, is stacked onto the second stacking unit 3B. As the number of sheets stacked on the second stacking unit 3B increases, the second stacking unit 3B is also lowered. When a full-state detection sensor 20 detects that the second stacking unit 3B is full of sheets, the lowering of the second stacking unit 3B is stopped.
The length of each sheet that is stacked as described above is, at the maximum, half (L/2) the length (L) of each sheet that is stacked over the entirety of both the first and second stacking unit 3A and 3B.
In the known sheet stacking apparatus 1, the position at which a sheet is discharged is limited to fixed positions where the first and second pairs of sheet discharging rollers 15 and 18 are disposed. Therefore, when sheets of an intermediate length such as a length of (3/4)L covering the entirety of the first stacking unit 3A and about half of the second stacking unit 3B are stacked in the sheet stacking apparatus 1, each of the sheets needs to be discharged through the first pair of sheet discharging rollers 15.
This makes the sheet stacking apparatus 1 handle sheets of an intermediate length with reference to the first pair of sheet discharging rollers 15, so that all of such sheets are stacked over at least the entirety of the first stacking unit 3A. As a result, for example, the user cannot freely stack such sheets at a position from which the user desires to carry the sheets.